1. Field of the Invention
The present invention relates to an electro-optic probe, wherein an electrical field generated by a measurement signal is coupled with an electro-optical crystal and light is made incident on this electro-optical crystal, and the waveform of the measurement signal is observed from the polarization of incident light. In particular the invention relates to an electro-optic probe with an improved optical system.
2. Description of the Related Art
By coupling with an electro-optical crystal, an electrical field generated by a measurement signal, and making a laser beam incident on this electro-optical crystal, the waveform of the measurement signal can be observed from the polarization of the incident light. Here the laser beam is in pulse form, and by sampling the measurement signal it is possible to measure at an extremely high time resolution. As a device which uses an electro-optic probe which utilizes this phenomenon, there is the Electro-optic sampling oscilloscope.
Compared with existing sampling oscilloscopes which use electrical probes, this electro-optic sampling oscilloscope (hereafter abbreviated as an EOS oscilloscope):
1) facilitates easy measurement since there is no need for a ground line when measuring a signal;
2) causes virtually no disturbance to conditions at the point of measurement, due to the high input impedance which can be realized because a metal pin at the tip of the electro-optical probe is insulated from the circuit system; and
3) is receiving attention because one of its features is that because it uses a light pulse, it is possible to measure a wide band through to GHz order.
However, as for the construction of conventional electro-optic probes which are used when carrying out signal measurement by means of an EOS oscilloscope, with the electro-optic element, it is necessary to adjust the plane of polarization of the incident laser beam with respect to the crystallographic axis. As a result, conventional technology probes must use a xc2xd wave plate in order to adjust the plane of polarization of the incident laser beam, the plane of polarization being adjusted by rotating the xc2xd wave plate about the optical axis.
The xc2xd wave plate, in addition to this property, has the property that with respect to the rotation angle of the xc2xd wave plate itself, the plane of polarization rotates through an angle twice this rotation angle. As a result, in order to adjust the rotation of the plane of polarization to the electro-optic element, it is necessary to have a mechanism for reducing the rotation with a worm gear or the like.
As a result of this, there is a problem in that the number of parts increases and so too does the cost. Furthermore, with more places for adjustment, the lower the usability and lesser the reliability. Furthermore, there is also the problem that with more optical parts, there is more unnecessary reflected light, so that the S/N at the time of measurement worsens.
Accordingly, one of the objects of the present invention is to provide an electro-optic probe that makes it possible to; decrease the number of parts and thereby make assembly easier, reduce both the cost and the number of adjustment places, increase reliability, and improve function by decreasing unnecessary reflected light.
With the present invention the object is achieved by providing an apparatus, namely an electro-optic probe comprising:
a laser diode for generating a laser beam based on a control signal from an oscilloscope body;
a collimator lens for making the laser beam into a parallel beam;
an electro-optic element having on an end face thereof a reflective coating, with optical characteristics which are changed by propagating an electrical field via a metal pin provided at the end face on the reflective coating side;
an isolator provided between the collimator lens and the electro-optic element, which passes a laser beam generated by the laser diode and isolates a reflected beam which is reflected by the reflective coating; and
photodiodes and which convert the reflected beam isolated by the isolator into electrical signals.
Accordingly, with the present invention, since it comprises an optical system where the xc2xd wave plate is omitted, it has the advantage that it is possible to decrease the number of parts and to reduce costs. Moreover, because the number of optical parts has been reduced, it has the advantage that the amount of unnecessary disturbed reflected light within the optical system is reduced.
Furthermore, with the present invention, because the xc2xd wave plate is omitted, and the electro-optic element is rotated directly, it is possible to adjust rotation without using a rotation adjustment mechanism such as a worm gear and the like. Therefore, assembly becomes easier, and ultimately it has the advantage of making it possible to increase reliability.